Packages for high frequency usages equipped with high frequency, high power semiconductors such as gallium arsenide field effect transistors have been used, for example, for RF (radio frequency) base stations. A high frequency semiconductor device generates a lot of heat when it is operating, so it may fail to work properly unless the generated heat is efficiently dissipated into the air. Therefore, a package for high frequency uses is typically equipped with a heat sink made of a substantially rectangular metal plate to provide a high heat dissipating characteristic with an area for mounting a semiconductor device. A ring-shaped frame member made of ceramics is joined on the outer periphery of the semiconductor mounting area of the heat sink plate. The space surrounded by the ring-shaped frame member and the heat sink forms a cavity for storing the semiconductor device.
After the semiconductor device is mounted on the heat sink plate, the upper surface of the ring-shaped frame is hermetically sealed with a cap member that seals the cavity. Also, external connection terminals are connected between the ring-shaped frame member and the cap member for signal input/output. The package for high frequency usages equipped with a sealed semiconductor is then affixed on the base plate to allow the heat transmitted to the heat sink to dissipate to the outside. Affixation is done by fastening the notched areas formed on both ends of the heat sink plate in the lengthwise direction using screws.
FIGS. 1(A) and (B) show a typical high frequency package 50 of the prior art. Number 51 denotes a heat sink plate having a thermal expansion coefficient close to that of the ceramic material made of a compound metal material with an excellent thermal dissipation characteristic such as copper-tungsten (Cu—W), and number 52 denotes a ring-shaped frame member made of a ceramic material such as alumina (Al2O3).
A metallic conductor pattern is formed on the front and back surfaces of ring-shaped frame member 52. The metallic conductor pattern on the back surface of ring-shaped frame member 52 and heat sink plate 51 are joined by means of brazing via Ag—Cu brazing metal 53. External connection terminals 54 are joined to the metallic conductor pattern on the surface of ring-shaped frame member 52 via Ag—Cu brazing metal 53. The brazing process is conducted by heating after placing ring-like frame member 52 on heat sink plate 51 via Ag—Cu brazing metal 53 and placing external connection terminals 54 on ring-like frame member 52 via Ag—Cu brazing metal 53. The metal surfaces of heat sink plate 51, ring-like frame member 52, and external connection terminals 54 are to be covered with metallic plating such as Ni plating and Au plating. A notch 57 is provided on each end of heat sink plate 51 in its longitudinal direction for fastening heat sink plate 51 to a base plate 55 with a screw.
Similar attempts of prior art include the kind where a sheet of indium is sandwiched between the heat sink plate and the base plate for improving thermal conductivity between the heat sink plate and the base plate, all of which are fastened together with screws (Japanese Laid-open Publication 2001-230349) or having a protrusion on each of the heat sink plate in its longitudinal direction for fastening with screws (Japanese Laid-open Publication H4-233752).
However, these packages of high frequency usages of prior art and their manufacturing methods have the following problems:
(1) While the heat sink plate and the external connection terminals are made of materials whose thermal expansion coefficients are similar to that of the ring-shaped frame member, it is difficult to match the thermal expansion coefficients of the heat sink plate and the external connection terminals perfectly with that of the ring-shaped frame member, so that it generates a stress in the joined area of high temperature brazing using Ag—Cu brazing metal. Consequently, a buckling occurs in the joined member. Since joined members with buckling that exceeds the tolerance limit cannot be used, it is a common practice to screen them against such a tolerance limit. More specifically, if the bottom surface of a heat sink plate, which is to be joined to the base plate, is caused to buckle in a concave shape, a space is created between the base plate and the heat sink when it is mounted on the base plate, thus affecting the heat dissipation characteristic. Therefore, it is necessary to select only those with convex buckling shapes. Thus, it mandates a selection process and lowers the yield, resultantly increasing the cost of the packages for high frequency uses.
(2) Using an indium sheet between the heat sink plate and the base plate brings about a cost increase for a package for high frequency usages because of the high cost of the indium sheet. Moreover, the use of an insertion such as an indium sheet makes the assembly process more complicated thus resulting in a cost increase for a package for high frequency usages.
(3) When a protrusion is provided at an end of the longitudinal direction of the heat sink plate, the package size is increased by the protrusion provided. For this reason, the need for miniaturization is not met.
(4) In forming the metallic plating film on the heat sink plate by means of a electrolytic plating method, the thickness of the heat sink plate on both ends tends to become too thick compared to the middle of the plate in its longitudinal direction, thus causing the bottom surface that is to be joined to the base plate of the heat sink plate to buckle into a concave shape. This generates a space between the heat sink plate and the base plate when it is mounted on the high frequency package on the base plate, resultantly affecting the thermal dissipation characteristics.
The present invention was made under the circumstance and is intended to provide an inexpensive package for high frequency usages having an excellent thermal dissipation characteristic.